1. Field of the Invention
This invention relates to a projection television (hereunder abbreviated as TV) in which an image formed on the screen of each of three cathode ray tubes, (hereunder abbreviated as CRTs) respectively corresponding to red, green and blue colors (hereunder referred to as red, green and blue CRTs, respectively) is magnified and projected therefrom on a rear projection screen (i.e., a large screen).
2. Description of the Related Art
FIG. 9 illustrates a conventional projection TV in which images formed on the screens of red, green and blue CRTs 1, 2 and 3 are respectively magnified by a lens 4 corresponding to red light, a lens 5 corresponding to green light and a lens 6 corresponding to blue light and are projected on a conventional two-sheet rear projection screen 34 which is a combination of two lens sheets, namely, a Fresnel lens sheet 7 and a lenticular lens sheet 8 as shown in FIG. 10.
Further, as illustrated in FIG. 10, black stripes 10 are provided on the surface, from which no light is utilized, of the lenticular lens sheet 8 of the conventional rear projection screen 34 as a countermeasure to prevent a deterioration of contrast measured under presence of external light irradiated thereon (hereunder sometimes referred to simply as "external-light contrast" or as "external-light contrast" ratio), which is caused due to reflection of external light thereon. The conventional rear projection screen 34, however, has a defect in that sufficient contrast cannot be obtained due to reflection of external light caused by a lens surface 11 of the lenticular lens sheet 8 or by an inner light-diffusing material of the sheet 8.
Thus, for the purpose of improving contrast, a further conventional rear projection screen has been developed, in which dye or pigment is added to the lenticular lens sheet 8. Further, there has been developed another conventional rear projection screen 35 wherein a lens sheet 9, into which dye or pigment is mixed (incidentally, such a lens sheet is referred to simply as a smoke sheet in the instant application), is provided in front of the lenticular lens sheet 8 as illustrated in FIG. 9, with the aim of decreasing reflection of external light and improving contrast. Such an improved conventional rear projection screen 35, however, has a drawback in that the optical transmittance (i.e., the screen gain) thereof decreases.
Results of measurement of performance (e.g., transmittance) of the conventional rear projection screens are shown in FIG. 11. As is seen from this figure, the transmittance of the conventional three-sheet screen 35 provided with the smoke sheet 9 having substantially uniform transmittance (e.g., 70%) in addition to the Fresnel and lenticular lens sheets is smaller than that of the conventional two-sheet rear projection screen 34 by 30%. Conversely, the "external-light contrast" of the screen 35 is greater than that of the screen 34 by 29%. The luminance of the screen 35, however, is 70.7% of that of the screen 34 (namely, is less than that of the screen 34 by 29.3%). Thus, there occurs a deterioration in luminance as indicated by a bar B of FIG. 7. As is apparent from the foregoing results, in the case of the conventional screen provided with not only the Fresnel and lenticular lens sheets but the smoke sheet, of which the transmittance is nearly uniform in the case where incident rays are visible ones, there inevitably occurs a deterioration in luminance in spite of increase in "external-light contrast".
A method for increasing CRT power has been proposed as a countermeasure to compensate the deterioration in luminance, which is caused by employing the smoke sheet in addition to the Fresnel and lenticular lens sheets. The transmittance of the smoke sheet 9, however, is nearly uniform in case that the wavelength of light impinging thereon is of visible region. It is, therefore, necessary for establishing the same color temperature (i.e., 9000.degree. K. corresponding to white) as of the conventional two-sheet screen 34 to increase beam currents respectively applied to the CRTs 1, 2 and 3 while a ratio of the magnitude of the beam current flowing through each of the red, green and blue CRTs to the total magnitude of the sum of the beam currents (hereunder referred to simply as a beam-current ratio) is maintained as in the case of employing the screen 34, which is indicated by a bar E of FIG. 8.
If a beam current, which is equal to a maximum current rating, flows through the blue CRT, the luminance of the blue CRT cannot be increased any more (incidentally, the beam-current ratios in the case of employing the conventional screen 34 is indicated by a bar D of FIG. 8).
Further, in the case where the conventional screen 35 is used, even if the beam current flowing through each of the red and green CRTs is less than or equal to a corresponding maximum current rating, the beam current flowing through each of the red and green CRTs cannot be increased if the beam current, which is equal to the maximum current rating, flows through the blue CRT. Thus, increase in luminance is not possible. Namely, the conventional screen 35 has a defect in that when the "external-light contrast" is increased, luminance decreases. The present invention is created to eliminate the defect of the conventional screen.
It is, accordingly, an object of the present invention to provide a projection TV which employs a wavelength-selection-type screen, the transmittance of which varies with the wavelength of light impinging thereon, and utilizes power of each CRT thereby minimizing the decrease in luminance when the "external-light contrast" is increased.